Telautograph system



K. w. MILLER 2,527,835

TELAUTOGRAPH SYSTEM Oct. 31, 19 50 2 Sheets-Sheet 1 Filed Nov. 4, 1947lA/VENTOR By k. w MILLER ATTORNEY Oct. 31, 1950 K. w. MILLER 2,527,835

TELAUTOGRAPH SYSTEM Filed Nov. 4, 1947 2 Sheets-Sheet 2 H IH 9, INVENTORfii-H-Mdk ATTORNEY K W MILLER l- JZWIII Patented Oct. 31, 1950 UNITEDSTATES PATENT". oFF eg TELAUTOGRAPH SYSTEM- Kenneth W. Miller, New York,N. Y., assignor' to Bell Telephone Laboratories, Incorporated; New York,N. Y., a corporationof'New York- Application November 4, l94-7v,; SerialNo. 783,902

has the unique properties of low resistivity together with uniformity ofresistance over the coated area and good characteristics for thetransmission of visible light.

The present invention is concerned with'the utilization of suchconductively coated glass sheet as a means for deriving electricalsignals representative of the position of a stylus or other writing suchas is necessary in telautograph and similar systems. The invention isalso concerned with the utilization in such systems of cathode ray tubesof the type having a crystalline target which is normally transparent tovisible light but which, when struck by an electron beam, form a depositwhich isopaque to visible light. Cathode ray tubes employing targets ofthat type are particularly adapted for projecting the images formed uponthe target upon an external screen and are also useful in applicationswhere it is necessary to retain the images formed for extended periodsof time. This latter characteristic is of interest in the art oftelautography where artificial signal storage means have been utilizedin the past in order to produce a recurring trace upon the screen of thecathode ray display tube. Finally, the invention is concerned with anovel combination of a coated glass sheet employed as a transmissiondevice With a cathode ray tube of the type described above wherein theimage received is projected upon the underside of the transparentwriting surface. Such' an arrangement has the advantage of providing ameans whereby an operator at either of two such stations may add to ormodify the graphic material being transmitted by'the other operator.

Accordingly. it is an object of the invention to provide an improvedsystem for the transmission of graphic information;

It is an object of the invention to provide a system for thereception-of graphic images which will retain the received images forrelatively extended periods of time;

It is an object of the invention to provide a novelmeans-of derivingpotentials representative 2 of" the position of a writingstylus on awriting surface;

It isan object of the inventionto provide a tela'utograph systemsuitable. for use in confer ence systems. f V

Theseand'otherobjects, features and aspects of the invention will bemore readily apparent from a' consideration of the following. detailedspecification and appended claim'takenin connection' with theaccompanying drawings, 'in" which:

Fig. 1 shows a schematic diagram of one S153! tion of a telautographsystem illustrating one embodiment of the invention; 7

Fig. 2 shows a schematic diagram ofa second station of av telautographsystem, illustrating alternative embodiments of the invention.

Referring now" to Fig. 1, there is shown a telautograph stationembodying a transmitting apparatus [0 for deriving signalsrepresentative of the position of a writing pencil or stylus intoelectrical signals for transmission to a remote station, and" areceiving apparatus Ijl for converting electrical signals received fromthe remote station into images representative of those being traced at'theremote station. Such a remote station i'sjshown in Fig. 2, wherein atransmitting apparatus I2 and a receiving apparatus l3 are essentiallysimilar to the corresponding apparatus of Fig. 1. I

The transmitting apparatus [0 of Fig. 1 is provided with a'sheet I4 oftransparent material having a thin, transparent,andielectric'ally'conductive coating on one side thereof which will bedesignated as a writing surface [5. Thesheet [4 may be of plate glasswhile the conductive co'atingmay be that knownas the NE'SA coating whichhas a resistance in the order of" to ohms per unit area. Such a coatedglass is described in the Transactionsof the American Institute ofElectrical Engineers, at page 789 of volume 65. j v

The sheet I4 is provided along two of itsisides with a pair ofelectrodes l6 and I1 which serve as terminals whereby"'alt'ernatingpotentials of a frequency fl from a source l8 may be. applied across theconductive coating in one coordinate direction. Similarly, there isprovided along'the' remaining two sides'a pairof electrodes l9 and 20whereby alternating potentials of afrequency )2' from asource 2| may beapplied across the conductive coating of the writing surface intheremaining coordinate direction. The voltage gradients thus producedacross the writing surface 15 are essentially mutually perpendicular;

at least within an area such as that enclosed by the dashed line 22, anda writing device 23 in contact with the writing surface will detectvoltages of frequencies fl and f2 whose amplitudes are proportional tothe two-dimensional coordinate position of the writing device as itmoves across the Writing surface. The writing device 23 may be a pencilor stylus adapted to provide an electrically conductive path as well asto produce visible marks upon the writing surface l5.

The voltages detected by the writing device 23 are applied to a bandpass filter 24 which is designed to pass onl the alternating potentialsof frequency H. The transmitted potentials are amplified by an amplifier25, rectified by a diode 26, and smoothed by a filter 21. The variableamplitude unidirectional potential thus obtained is utilized to modulatea carrier wave of frequency f3 supplied by a source 28 to a modulator 29and the modulated carrier transmitted over a wire or radio transmissionchannel 30 to a remote receiving station such as the receiving apparatusl3 of'Fig. 2. Similarly, the potentials of frequency f2 detected by thewriting device 23 will bepassed by a band pass filter 3| and amplifiedby an amplifier 32. The amplified potentials are converted into avariable amplitude unidirectional potential by a rectifier 33 and afilter 34, and'the resultant wave utilized to modulate a carrier wave offrequency f4 supplied by a source 35150 a modulator 36. The modulatedcarrier is then transmitted over a wire or radio transmission channel 31to the remdte receiving station. It will be realized that, since theonly information transmitted is that representative of the instantaneousposition of the writing stylus 23, the transmission channels 30 and 37may be of relatively narrow bandwidth.

The receiving apparatus includes a cathode ray tube 38 having means togenerate an electron beanrsuch as a conventional electron gun 39, meansfor deflecting the electron beam, such as a pair of horizontaldeflection coils 40 and a pair of vertical deflection coils 4|, and atarget 42, The target 42 consists of a crystalline layer 43 provided oneach side with electrodes 44 and 45 in the form of thin, transparent,sputtered metallic layers or fine meshes. The target 42 is maintained ata positive potential with respect to the electron gun 39 by a source 46while the electrode 45 is maintained at a potential positive withrespect to the electrode 44 by a source 41.

The crystalline layer 43 of the target 42 is composed of crystals of theclass known as ionic crystals which are normally transparent to visiblelight but, upon being struck by an incident beam of electrons, form anopaque deposit of a densit proportional to the instantaneous densitycf'the beam. This opaque deposit persists after the beam leaves the areaand moves transversely through the crystal toward the positive electrode45 at a speed determined by the temperature of the crystal and thepotential difference between the electrodes 44 and 45. These effectsoccur primarily in crystals of the alkali-halides, such as potassiumchloride. For a further description of the phenomena, reference may bemade particularly to an article entitled A system of Large-ScreenTelevision Reception Based on Certain Electron Phenomena in Crystals, byA. H. Rosenthal and published in the'Proceedings of the Institute ofRadio Engineers for May, 1940, at page 203.

' The electron beam from the electron gun 38 is deflected over thesurface of the target 42 to produce an opaque image of the graphicmaterial being drawn at the remote transmitting station upon thenormally transparent target. Carrier waves modulated in accordance withsignals representative of the horizontal coordinate position of thewriting device at the transmitting station are received over atransmission channel 48 whence they are demodulated by a detector 49.The potentials so derived are amplified by a direct-coupled amplifier 50and applied to the defiection coils 4|! to produce a horizontaldeflection of the electron beam proportional'to the horizontal movementof the distant writing device. Similarly, carrier waves modulated inaccordance with signals representative of the vertical coordinateposition of the writing device at the transmitting station are receivedover a transmission channel 5| and demodulated by a detector 52. Thepotentials so derived are amplified by an amplifier 53 and applied tothe deflection coils 4| to produce a vertical deflection of the electronbeam proportional to vertical movement of the distant writing device. Itis to be understood that the intensity of the moving electronbeam is tobe maintained at a constant level such that an opaque image of theproper density is formed upon the target 42.

In accordance with one feature of the invention, the image formed uponthe target 42 is projected upon the under surface 54 of the sheet M bymeans of light from a source 55 which passes through the target toproduce a reproduction of the target image upon the surface 54. It is tobe understood that the projection system illustrated is symbolic only,since it will usually be necessary for the rays of light from the source55 to pass through the target in a direction normal to the surfacesthereof over the entire target.

It is an important aspect of the invention that the target 42 has a highstorage ratio, that is, the image formed upon the screen is retained fora considerable period after formation by the moving electron beam. Thiseffect is due to the fact that the opaque deposits produced by theelectron beam move transversely through the crystal at a relatively slowrate determined by the potential normally applied between the targetelectrodes 44 and 45. The necessity for the use of artificial signalstorage means such as have been necessary in the past in telautographsystems utilizing the usual type of cathode ray tube for displaypurposes is thus obviated. It is, however desirable that a means heprovided whereby the operator can erase the image at will. Accordingly,there is provided a switch 55 whereby a relatively high potential may beapplied across the electrodes 44 and 45, thereby temporarily hasteningthe movement of the opaque deposit toward the positive electrode andcausing the rapid dissipation of the recorded image. Alternatively,other known means, such'as increasing the temperature of the crystal 42,may be utilized to perform the same function.

Referring now to Fig. 2, the remote station illustrated includes atransmitting apparatus l2 connected by means of the transmissionchannels 48 and 5| to the receiving apparatus of Fig; 1. Similarly, thereceiving apparatus l3 of Fig. 2 is connected by means of thetransmission channels 30 and 31 to the transmitting apparatus ll! ofFig. 1. While the receiving and transmitting apparatus 2 and 13 of Fig.2 is essentially the same as the corresponding apparatus of Fig. 1,alternative embodiments of certain aspects of the respective circuitsare provided and will be described in some detail.

The transmitting apparatus [2 includes a sheet 5'! of transparentmaterial having a, writing surface 58 which includes a thin,transparent, electrically conductive coating similar to that of thesheet I of Fig. v 1. Alternating potentials of frequency f5 are appliedto the conductive coating along two opposite sides of the sheet 51 bymeans of a pair of electrodes 66 and SI while alternating potentials offrequency f6 from a generator 62 are applied to the conductive coatingalong the remaining two sides of the sheet 5'! by means of a second pairof electrodes 63 and 64. A po tentiometer 65 connected across thegenerator 59 together with the conductive coating of the sheet 51combine to form a bridge circuit which becomes more or less unbalancedin accordance with the horizontal position of a writing stylus 66, thusimpressing upon a band pass filter 61 potentials of frequency f5 whoseamplitude varies in accordance with the horizontal position of thewriting stylus 66 on the writing surface 58. Similarly, a potentiometer68 connected across the generator 62 together with the conductivecoating of the sheet 51 combine to form a bridge circuit which impressespotentials of frequency f6 and amplitude proportional to the verticalposition of the writing stylus upon a band pass filter 69. The variableamplitude alternating potentials of frequency f5 passed by the filter 61and the variable amplitude alternating potentials of frequency f6 passedby the filter '69 are utilized to modulate carriers for transmissionover the transmission channels 48 and 5| after the same fashion as isemployed in the transmitting apparatus In of Fig. l.

The alternative circuit just described is advantageous in that thewriting stylus '66 may be maintained at ground potential, thus obviatingany possibility of even a slight discomfort to the user due topotentials existing between the pencil and the writing surface. However,the bridge circuits including the potentiometers 65 and 68 shouldpreferably be adjusted for balance condition when the writing instrument66 is in contact with some extreme point in both coordinate directions,such as the point designated as 10, in order that difficulties due tophase reversals of potential are not encountered at the balance point.

The receiving apparatus l3 utilizes the principle that a target of thenature described, that is of alkali halide crystals, when scanned by anelectron beam of high intensity so as to produce an opaque deposit ofhigh density and subsequently scanned by a second beam of lesserintensity, will have the opaque deposit produced by the first scanningbeam reduced in density to a level corresponding to the intensity of thesecond scanning beam. There is accordingly provided in the receivingapparatus IS a cathode ray tube H similar to the tube 38 of Fig. I,having an electron gun 12, and a target 13 composed of a layer of alkalihalide crystals 14 and a pair of supporting electrodes 15 and 16. Acontrol electrode 11 of the electron 12 is supplied through a switch 18by a potential source 19 so that, when the switch 18 is in a firstposition A, the electron beam is of such an intensity as will produce anopaque deposit in the crystal 14 of a density suificient to preclude thetransmission of light therethrough. When the switch 18 is in a positionB the electron beam is of an intensity such as will reduce the densityof the opaque deposit to allow the free transmission of light. Anelectron beam deflection system comprising the usual deflection coils B9and BI is also connected through the switch 78 so as to be maderesponsive either to incoming signals from the transmission channels 30and 31 or to signals from a scanning wave generator 82. The scanningwave generator 82 may be of a well-known type which-will producedeflection currents for deflecting the electron beam in a raster ofclosely-spaced scanning lines so as to produce a uniform opaque depositover the entire surface of the crystal 14 when the switch 18 is placedin position A. Thus, when the apparatus I3 is made responsive toincoming signals by the adjustment of the switch 18 to position B, theelectron beam will form an image upon the target 13 which is transparentto visible light. The image then projected upon the sheet 51 by lightfrom a source 83 will be bright upon a dark background rather than darkupon a bright background as in the case of the receiving apparatus ofFig. 1.

In the operation of the invention in the intended fashion, the graphicmaterial drawn on the writing surface of the transmitting apparatus ofone station is reproduced as an image upon the underside of the writingsurface at the receiving station. The person at the receiving stationmay therefore, by making appropriate markings upon his writing surface,effectively add to or modify the original sketch. It will thus be seenthat the system of the invention is particularly adaptable to use inconference systems wherein it may be desired to exchange ideas orinformation involving graphic or other symbolic data.

What is claimed is:

In combination, a writing surface composed of a sheet ofmaterial havinga uniform resistance over an exposed area, a first generator foralternating potentials of a first frequency, means for applyingpotentials from said first generator across said writing surface in onecoordinate direction, a potentiometer connected across said firstgenerator said potentiometer having a tap, a band pass filter having aninput circuit connected to said tap, a second generator for alternatingpotentials of a second frequency, means for applying potentials fromsaid second generator across said writing surf-ace in a secondcoordinate direction, a potentiometer connected across said secondgenerator, said potentiometer having a tap, a band pass filter having aninput circuit connected to said last-named tap, an electricallyconductive writing stylus in contact with said writing surface andconnected to input circuits of the two said band pass filters wherebythe single frequency potentials passed by each filter have amplitudesrepresentative of the coordinate positions of the said writing stylus.

KENNETH W. MILLER.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,000,014 Du Mont 1 May 7, 19352,168,049 Skellett Aug. 1, 1939 2,227,083 Handrick Dec. 31, 19402,338,949 Kupfmuller et a1. J an. 11, 1949

